1. Field of the Invention
The present invention relates to display devices and electronic devices.
2. Description of the Related Art
In recent years, display devices using liquid crystal panels and display devices using organic EL panels have been under active development. These display devices are broadly classified into display devices in which only transistors for pixel control (pixel transistors) are formed over a substrate and scanning circuits (driver circuits) are included in peripheral ICs and display devices in which scanning circuits are formed over the same substrate as pixel transistors.
Display devices in which driver circuits are integrated with pixel transistors are effective for reduction of the frame width of the display devices or cost of the peripheral ICs. However, transistors used in the driver circuits are required to have better electrical characteristics (e.g., field-effect mobility (μFE) or threshold voltage) than the pixel transistors.
Although a silicon-based semiconductor material has been widely known as a material for a semiconductor thin film applicable to transistors, an oxide semiconductor is attracting attentions as another notable material, (e.g., Patent Documents 1 and 2). For example, a transistor using the following amorphous oxide is drawing attentions: the amorphous oxide contains indium (In), gallium (Ga), and zinc (Zn) and has an electron carrier density of less than 1018/cm3.
Transistor using oxide semiconductors for semiconductor layers have higher field-effect mobility than transistors using amorphous silicon of silicon-based semiconductor materials for semiconductor layers. Hence, the transistors using oxide semiconductors can operate at high speed and be suitably used for the display devices in which pixel transistors are integrated with driver circuits. Besides, a manufacturing process of such transistors using oxide semiconductors is easier than a manufacturing process of transistors using polycrystalline silicon for semiconductor layers.
However, a problem of the transistors using oxide semiconductor for semiconductor layers is that entry of impurities such as hydrogen or moisture into the oxide semiconductor generates carriers and changes threshold voltages as one of electrical characteristics of the transistors.